Photographic colour printing



Nov. 1, 1966 b. M. NEALE 3,282,190

PHOTOGRAPHIC COLOUR PRINTING Filed Oct. 18, 1963 5 Sheets-Sheet 1 FIG! Nov. 1, 1966 D. M. NEALE PHOTOGRAPHIC COLOUR PRINTING 5 Sheets$heet 2 Filed Oct. 18, 19 3 QUE N QR um QWN Nov. 1, 1966 D. M. NEALE PHOTOGRAPHIC COLOUR PRINTING 5 Sheets-Sheet 5 Filed 001;. 18, 1963 vom i la United States Patent PHOTOGRAPHIC COLOUR PRINTING Denis Manktelow Neale, Ilford, Essex, England, assignor to Ilford Limited, Ilford, Essex, England, a British com- P y Filed Oct. 18, 1963, Ser. No. 317,190 Claims priority, application Great Britain, Oct. 25, 1962, 40,486/ 62 2 Claims. (Cl. 95-73) This invention relates to photographic printing and in particular to the printing of multicolour photographic transparencies by optical projection on to photosensitive print material.

When multicolour transparencies, and colour negatives in particular, are printed on to multicolour photosensitive print material, the relative exposures of the print material to red, green and blue light must be adjusted so that the print after processing to colour is free from any disagreeable colour cast. The adjustment of these exposures may be efiected by controlling the relative intensities of red, green and blue light used simultaneously to expose the photosensitive print material. A practical difficulty is found in measuring accurately these relative intensities for each of a number of colour negatives of differing density and colour. If a photo-electric device is used to measure the intensity of each colour in turn, the adjustment of intensity ratio by insertion of colour-selective filters becomes very tedious. This is because practical filters generally show substantial unwanted absorption in those bands they are intended to transmit freely. Thus, for example, insertion of a magenta filter produces a reduction not only in the intensity of green light but also in the intensity of blue light due to the unwanted blue absorption of any practical magenta. Similarly a cyan filter may afi'ect green and blue light as well as red.

The establishment of a satisfactory proportion of red, green and blue light intensities is greatly simplified if the ratio of intensities of two colours may be assessed by a Single operation. As a filter is adjusted to modify the intensity of light of one colour, its eiiect on a second colour is thereby taken into account. If the ratio of red and green light intensities is adjusted by adding or removing a cyan filter, for example the green absorption of the cyan filter is taken into account. The intensity of blue light may be affected by adjustment of the cyan filter, but a subsequent adjustment of blue light intensity by a yellow filter will not greatly disturb the red/ green intensity ratio already established.

' The advantages of measuring the ratio of intensities of two colours have been stated already by Calkin, Hunt and Letzer (Photographic Science and Engineering, vol. 5, No. 6, pp. 3664373, November/December 1961). The means used to effect this ratio measurement have hitherto been rather unsatisfactory, however. The method described by Calkin, Hunt and Letzer involves balancing the outputs from two photocells. The sensitivity to a change in intensity ratio is thus proportional to the intensity level at which colour assessment is performed. As a result, their, method is not very suitable for use on the print plane of a vertical enlarger since the intensity level at the print plane varies greatly with the magnification to which the enlarger is set. Furthermore, the sensitivities of practical photocells vary s-ufficiently from day to day to give rise to significant errors in the establishment of a required intensity ratio between two colours. It is therefore preferred to use a single photocell for the assessment of an intensity ratio of two colours. A change in photocell sensitivity will not then affect the assessment of intensity ratio provided the special sensitivity of the photocell remains unchanged, i.e. the sensitivity to any one wavelength remains in a fixed proportion to the sensitivity at any one other wavelength.

If the photocell is used in a circuit arrangement providing a logarithmic relation between electrical output and incident light intensity, the sensitivity becomes substantially independent on enlarger magnification. Thus, for example, light of two colours A and B may be allowed to fall alternately on the photocell, producing output voltages E and E respectively. If the photocell currents corresponding to the two colours are respectively I and 1 and if the photocell delivers a current proportional to incident light intensity, then it follows that E E =Log I Log I =Log (g) The difference between the output voltages is thus an indication of the ratio of photocell currents corresponding to the two colours.

In practice it is inconvenient to have to establish which is the greater, E or E Also, the accurate determination of (E -E can be inconvenient when (E -E can pass through zero. It is therefore proposed to illuminate the photocell continuously with light of colour A, but cyclically to interrupt illumination by light of colour B. The alternating component E in the output from the logarithmic amplifier thus represents (E 'E and accordingly is of a constant polarity. Moreover I as This means that, provided the response to the cyclically interrupted light is never less than ten times the response to the uninterrupted light, the output voltage E is directly proportional to the logarithm of the ratio of intensities of the two colours, A and B.

According to the invention, therefore, a method of making from multicolour transparencies prints on photographic material containing at least two components selectively sensitive to two different spectral bands, comprises allowing printing light to fall on a photocell, cyclically interposing in the path of said light a filter passing one of said spectral bands but not the other, passing current from said photocell through a logarithmic amplifier, adjusting the relative intensities of light in the two spectral bands to an adjusted proportion which produces in the output of the logarithmic amplifier an alternating component of current of predetermined amplitude and exposing said photographic material by printing light having relative intensities of light in the two spectral bands in said adjusted proportion.

It is generally appropriate to regard the spectrum as split into three bands, red, green and blue and to use any two of these in the aforesaid process. The preferred pairs are red and green, and red and blue.

Thus, if I I For successful operation of the method, a combination of photocell and logarithmic amplifier is required providing an accurate logarithmic response to incident light over a very wide range of intensities. It has been found that this requirement is met by the circuit arrangement described by Hariharan and Bhalla (J. Sci. Inst., 33, No. 2, pp. 69-71, 1956).

According to a particular form of the invention, therefore, the photocell is of the electron multiplier type and is operated at substantially constant collector current, part of the dynode supply voltage to said electron multiplier being applied to a logarithmicamplifier from the output of which an alternating voltage is obtained which is used as a measure of the ratio of intensities of light of the colours. in the two spectral bands.

A particular embodiment of the invention will now be described with reference to the accompanying drawings in which:

FIG. 1 represents a side elevation of the photocell unit and associated colour selective filters.

FIG. 2 represents a plan view of the same unit.

FIG. 3 represents the electrical circuit of the photocell power supply and logarithmic amplifier.

FIG. 4 represents the A.C. amplifier and meter circuits.

The circuits of FIGS. 3 and 4 constitute a single circuit when that of FIG. 3 is placed to the left of that of FIG. 4.

Referring to FIGS. 1 and 2, light reaching the sidewindow multiplier photocell 1 must first pass the stationary colour selective filter 2 and the sector disc 3 rotated by the motor 4. The filter 2- is an Ilford No. 109 Yellow which therefore passes the green and red components of printing light but absorbs the blue. The sector disc 3 rotates at 3,000 r.p.m. and comprises two transparent sectors 5 and 6 of 90 each and two sectors 7 and 8 also of 90 each which are covered with Ilford filter No. 204 Red.

As the disc rotates, therefore, red light may at all times pass freely through yellow filter 2 and the disc to reach the multiplier photocell 1'. Green light passing the yellow filter 2 is modulated by rotation of the disc 3, since sectors 5 and 6 transmit green light freely, but sectors 7 and 8 absorb it. It follows, therefore, that the photocell is alternately receiving red light alone and red light and green light simultaneously. This is the condition of the apparatus providing an indication of the ratio of green light to red light.

When it is required to measure the ratio of blue light to red light, filter 2 is displaced to the position shown dotted as hand the magenta filter 9 (Ilford N0. 502) is I slid into the operative position formerly occupied by the yellow filter 2. When this is done, magenta filter 9 prevents green light from reaching the photocell. Blue light passing the magenta filter 9 is then modulated by rotation of the disc 3, whereas red light reaches the photocell substantially without modulation.

The current emitted by the cathode of photocell 1 comprises a DC. component corresponding to the unmodulated red light intensity and an A.C. component at 100 c./s. corresponding to the modulated green or blue light reaching the cathode. The photocell is'connected in the circuit shown in FIG. 3. This is based closely on the arrangement described by Hariharan and Bhalla and its function has been described by them (100. cit.). The circuit of FIG. 3 continuously controls the amplification of the multiplier photocell so that the collector current of the photocell is substantially constant at any instant. To do this it varies the photocell supply voltage in sympathy with the incident light intensity. Part of the photocell supply voltage is applied through a current limiting resistor to the grid of valve V7 which functions as a logarithmic amplifier. Hariharan and Bhalla have shown that the output from this amplifier is a logarithmic function of the light intensity incident on the photocell over a range of intensities of 10 :1.

The alternating component of the output of the logarithmic amplifier is applied to the grid of the tuned amplifier V8. The anode load of this amplifier is a circuit tuned to the fundamental frequency at which the green or blue light is being chopped, viz. 100 c./s. From the .tuned amplifier the 100 c./s. signal passes to a negative feed-back amplifier comprising valves V911 and V9b.

The A.C. output of this amplifier is rectified by diodes MR3 and MR4 to produce across R31 a DC. voltage proportional to the A.C. output of the negative feed-back amplifier. The cathode follower V10a applies this DC. voltage to one side of the meter M. The second cathode follower V10b applies to the other side of the meter a potential determined by the settings of potentiometers RV7 and RV8 or RV9.

As the instrument embodies a logarithmic amplifier, the potentiometers RV7, RV8 and RV9 may be marked with linear scales in terms of colour density units. Thus the green control, RVS, may be calibrated to indicate positions which will maintain a given deflection on the meter M as dilute green-absorbing (magenta) filters of 1 equal strength are inserted in the path of light reaching the multiplier photocell 1. Conveniently, the marked increments will correspond to the successive addition of filters of green-light density 0.05. Similarly the blue control RV9 will be marked in increments of yellow density and the red control in increments of cyan density. In a typical application, several trial prints may be made I from a particular test negative using different combinafirst placed in the enlarger with the combination of colour correction filters chosen for the test negative. Light of printing quality is allowed to fall on to the photocell unit. The yellow and magenta filters (2 and 9 respectively) are set to the position shown in FIG. 2, i.e. with the yellow filter 2 in operative position over the multiplier photocell 1. With switch S1 in the position shown in FIG. 4 the green/red zero control RV4 is now adjusted to bring the meter indication to zero, thereby indicating equality in potential of the cathodes of V10A and V10B. The yellow filter 2 is now moved to position shown at 2a in FIG. 2 and the-magenta filter 9 is thereby brought into operative position above the photocell 1. Switch S1 is now moved to the alternative position and the blue/red zero control RVSis adjusted to return the meter M once more to zero indication.

The negative to be printed is now replaced by the test negative. With yellow filter 2 in operative position and switch S1 in the position indicated, the green control RV8 may be adjusted to restore meter M to zero indication. Yellow filter 2 is now removed from operative position and replaced by magenta filter 9 and switch S1 is moved to thealternative position. The blue control RV9 is now adjusted to restore meter M to zero indica- -tion. The new combination of settings of RV7, RV8 and way after RV8 has been adjusted to suit the negative being examined, it is then necessary to return filter 2 to operative position, return switch S1 to the position indicated and readjust RV8.

An alternative method of using the apparatus comprises adjusting the controls RV4, RVS, RV7, RV8 and RV9 as described above so that the meter provides zero indication with either the yellow or the magenta in operative position before the multiplier photocell and S1 in the appropriate corresponding position. When this result has been obtained for the particular test negative and the combination of colour correction filters which provide a satisfactory print therefrom, an alternative negative may be substituted for the test negative. The colour correction filters used in the lamphouse may then be adjusted until the meter M again indicates zero with either filter 2 or filter 9 in operative position and with switch S1 in the corresponding condition as described above.

In FIG. 3, the controls RV1 and RV2 are provided to allow the operating conditions of V7 to be adjusted to provide a logarithmic response over the widest possible range of photocell supply voltage.

In FIG. 4, the control RV3 provides adjustment of the degree of negative feedback used in the feedback amplifier V9A and V9B. 'It controls the amplification of the amplifier and therefore the sensitivity of the instrument.

RV6 controls the current through the potentiometer chain formed by RV7, RV8 and RV9. By adjusting RV6, it is thus possible to adjust the sensitivity of controls RV7, RV8, RV9 to conform to the sensitivity of the instrument.

To restrict the response of the photocell 1 to the appropriate spectral bands, it is preferred that a didymium glass filter (2 mm. of Chance ON 16) should be placed in the path of light reaching the photocell. This filter is indicated in FIGURE 1 by the numeral 10.

It will be appreciated in connection with the foregoing description of the apparatus that a photomultiplier (multiplier photocell) is a combination of a photo-emissive photocell and an electron multiplier contained in the same envelope. The electron multiplier is essentially a linear current amplifier butin conjunction with known arrangements of other'circuit components it may form part of a logarithmic amplifier operating over a dynamic range of 10 to 1.

The following is a list of the components shown in the circuits of FIGS. 3 and 4:

Value or Circuit reference identification R1 5 K R2 47 K R3 47 K R4 10K R5 ohms R6 330 K R7 47 K R8-R15 8 x 47 K R16 100 M R17 10 K R18 30 M R19 4.7 K. R20 100 K. R21 5 ohms. R22 1 M. R23 330 ohms R24 1.5 K. R25 680 K. R26 4.7 K R27 22 K R28 47 K R29 4.7 K R30 1 M R31 1 M R32 47 K. R33 100 K.

RV1 10 K RV2 10 K RV3 2 M RV4 100 K.

RV5 100 K.

V' 66 K.+66 K. RV8 136 K.

RV9 136 K.

C2 l,u.f

C3 0.1 ,uf

c4 0.1 .af.

C5 l6 ,uf (electrolytic) C6 0.2 ,uf

C9 0.1,uf.

C11 4 ,uf (electrolytic). C12 0.5 ,tr.

MR1 Rectifier.

MR2 Rectifier.

MR3 ZS 74 (rectifier). MR4 ZS 74 (rectifier). V1 s 75/20.

V3 EL 360.

V4 EF 86.

V7 ECC 83.

V8 EL 91.

V9 -ECC 82.

V10 ECC 82.

L1 12 H (choke). 1 M1 50-040 ,u.A. (meter), S1 TWO pole changeover switch. T1 Transformer.

I claim as my invention:

1. A method of making from multicolour transparencies prints on photographic material containing at least two components selectively sensitive to two different spectral bands, which comprises allowing printing light to pass to a photocell through a didymium glass filter thereby restricting the response of the photocell to the said spectral bands; cyclically interposing in the path of said light a filter passing one of said spectral bands, but not the other, to said photocell; passing current from said photocell through a logarithmic amplifier, adjusting the relative intensities of light in the two spectral bands to an adjusted proportion which produces in the output of the' logarithmic amplifier an alternating current component of predetermined amplitude; and exposing said photographic material by printing light having relative intensities of light in the two spectral bands in such adjusted proportion.

2. A photographic printing apparatus comprising a printing light having two spectral bands of different colors, filter means which are cyclically interposed in the path of said printing light whereby one of the color bands but not the other can be transmitted, a single photocell of the electron multiplier type which operates at substantially constant collector current, said photocell being disposed so that the light transmitted through said filter means will be detected by said photocell, a logarithmic amplifier coupled to said photocell so that the output of said photocell will be operably coupled to said amplifier, circuit means for feeding a portion of the dynode supply voltage of said photocell to said logarithmic amplifier, adjusting means for establishing the relative proportion of printing light in the two spectral bands which reaches said photocell and measuring means to determine the alternating current component output of said logarithmic References Cited by the Examiner UNITED STATES PATENTS Tuttle 88-24 Varden 88-24 Simmon 8824 Hirsch 88-24 X Simmon et a1. 95-73 8 OTHER REFERENCES A Wide Range, Recording, Logarithmic Photometcr Circuit by Hariharan and Bhalla, published in the Journal of Scientific Instruments, vol. 33, N0. 2, pages 69-71, February 1956, copy in Scientific Library Q 184.17.

JULIA E. COINER, Primary Examiner.

IEYON c. BLUNK, Assistant Examiner. 

1. A METHOD OF MAKING FROM MULTICOLOUR TRANSPARENCIES PRINTS ON PHOTOGRAPHIC MATERIAL CONTAINING AT LEAST TWO COMPONENTS SELECTIVELY SENSITIVE TO TWO DIFFERENT SPECTRAL BANDS, WHICH COMPRISES ALLOWING PRINTING LIGHT TO PASS TO A PHOTOCELL THROUGH A DIDYMIUM GLASS FILTER THEREBY RESTRICTING THE RESPONSE OF THE PHOTOCELL TO THE SAID SPECTRAL BANDS; CYCLICALLY INTERPOSING IN THE PATH OF SAID LIGHT A FILTER PASSING ONE OF SAID SPECTRAL BANDS, BUT NOT THE OTHER, TO SAID PHOTOCELL; PASSING CURRENT FROM SAID 